Data storage devices employ rotating data storage media such as hard disk drives. In a hard drive, data is written to the disk medium using a write head which generates a high localized magnetic field which aligns magnetic domains within the disk in one of two directions. In some cases, the magnetization direction is up or down relative to the plane of the disk (perpendicular magnetic recording, or PMR). In other cases, the magnetization direction is within the plane of the disk. In all cases, this data may then be read-out with a read head. The write and read heads are typically integrated within a single assembly. To achieve steadily increasing data storage densities (typically measured in bits/inch2), which are now achieving levels near 1012 bits/in2, the sizes of magnetic regions storing individual bits have been reduced to nm levels. Writing to, and reading from, such small regions may include shrinking the sizes of the read and write heads and also having them “flying” closer to the disk surface (since the magnetic forces drop rapidly with increasing distance between the disk and the head). The distance between the head and the disk is called the “flying height” since the head is said to “flying” above the disk on a cushion of compressed air which is entrained by the rapid rotation of the disk and then squeezed between the head (often called a “sled”) and the disk. Very precise control of the flying height is achieved using “thermal flying height control” (TFC) which employs an electrical heater (with mW powers) to heat the pole pieces of the head, resulting in nm-level thermal expansion which pushes the pole pieces slightly closer to the spinning disk surface. In more recent hard drive data storage devices, such as that described in U.S. Pat. No. 7,990,647 B2, issued Aug. 2, 2011, the read/write head may also incorporate a near field light source or a microwave source. In the following description of embodiments, “light source” may be interpreted to also comprise “microwave source”, and the term “Heat Assisted Magnetic Recording” (HAMR) may be interpreted to also comprise “Microwave Assisted Magnetic Recording” (MAMR). One purpose for this light source may be to locally heat the disk medium, thereby momentarily lowering the coercivity and thus reducing the required writing current. This writing process is known as “Heat Assisted Magnetic Recording”, or HAMR. However, near field light sources are typically only 10% efficient in heating the disk medium, with the remainder of the laser power heating the write head. Thus the laser light source represents another potential method for heating the write head in order to increase protrusion and reduce the flying height for improved writing. There are three different sources of heat which may heat the write head, where all of these sources of heat may cause the pole pieces to expand and thus reduce the flying height of the write head above the disk medium: 1) the current flowing through the write head coil, 2) the current flowing through the flying height control heater, and 3) the majority (˜90%) of the power flowing into the near field light source. Each of these three sources of heat to the write head has a different time constant for heating up (when power is applied) and for cooling down (when power is removed).
A goal of some embodiments is to provide a method for improved control of the head protrusion in a heat assisted magnetic recording or microwave assisted magnetic recording disk drive.
A further goal of some embodiments is to provide pre-heating and feed-forward control to increase the head protrusion during a pre-heating operation which precedes a writing operation.
A still further goal of some embodiments is to use feed-forward control of the power going into the head during the pre-heating operation.
Another goal of some embodiments is to use feed-back control of the media heater power during a writing operation to reduce the laser power decrease arising from heating of the laser diode used in an HAMR process.